Users of instruments for the sensitive detection and analysis of gamma and neutron radiation often face difficulties with reliable verification of the instrument performance and calibration that typically requires the usage of suitable gamma and neutron check sources. This may not be a big obstacle for radionuclide laboratories and users in the nuclear industry, but other users, such as law enforcement officers, do not have suitable means to regularly test and verify the performance of their radiation detection equipment. Sending the equipment to a calibration laboratory corresponds to a significant increase in the cost of ownership and a reduction of the availability of the instrument. In particular, personal radiation detectors (PRD) or spectroscopic radiation detectors (SPRD) that are designed to be used by law enforcement personnel, fall under the described problem regarding the missing presence of suitable man-made check sources.
Regarding the gamma radiation detector, there are some established solutions using natural radioactivity (primordial isotopes). For example, natural Lutetium test adapters containing primordial Lu-176 or natural Potassium containing primordial K-40 can be used as a safe way to verify the performance of PRD or SPRD devices. See U.S. Pat. No. 7,544,927, hereby incorporated by reference in its entirety (however, where anything in the incorporated reference contradicts anything stated in the present application, the present application prevails). No such natural radionuclides, however, exist to test the neutron detector. Ownership of a man-made neutron source on the other hand is prone to significant administrative problems and may as well relate to radiation protection issues. Consequently, the neutron detector in such PRD and SPRD devices is usually checked very infrequently or not at all. Unlike the gamma radiation background of typically 5 to 50 counts per second (cps), the neutron radiation background count rate of those devices is extremely low (<<1 cps), so operators are accustomed to seeing a zero count rate in the display of those devices and may not get alerted by a defective neutron detector. Manufacturers can set a neutron fail rate threshold in the instrument, but the neutron background during operation may be locally extremely low (e.g., for usage in an underground location such as a metro system). Therefore, in order to avoid a false failure message of the instrument, this state-of-health indication may only detect a total failure of the detector (e.g., several hours with no count).
There is, therefore, a need for a method to verify the operational status and to calibrate the sensitivity of neutron detectors, such as portable neutron detectors, without usage of a conventional neutron source such as AmBe or Cf-252.